Meraklis behind the harbour; Dimitris at the bottom of the school steps; Pizza and Pasta (Dolphin) just over the bridge at the harbour; Trawler - harbour side are all reasonably priced and have a wide price range across quite a wide menu. As a guide we normally pay around �30 to �35 in these places (Dolphin closer to �25) for the two of us with starter and main course, water, bread, and a half kilo retsina. Many places on Symi provide a complementary piece of fruit or cake to finish your meal.
Gyros is great value for money.
Manos Fish Restaurant at the harbour side can be very expensive although there is always a great atmosphere there.
Unlike many British establishments prices and menus are readily shown at tavernas so you don't have to go in sit down and then leave when you find the offerings don't suit.
Have fun